Many colloidal silicas consisting of non-spherical silica particles have been proposed. JP1-317115A discloses a stable silica sol containing amorphous colloidal silica particles dispersed in a liquid medium, the particles having an elongated shape elongated in only one plane and a uniform diameter along the elongation within the range of 5 to 40 mμ as measured by electron microscopic observation. JP 4-187512 A discloses a silica sol consisting of silica particles having an elongated shape, which are obtained by a method comprising adding a metal compound such as an aluminum salt, before, during, or after a step of adding a silicic acid solution. JP 11-60232 A discloses colloidal silica consisting of cocoon-shaped silica particles having a ratio of long axis to short axis of 1.4 to 2.2, which are obtained by hydrolyzing alkoxysilane. JP 2001-48520 A describes that colloidal silica containing non-spherical silica particles obtained by an active silicic acid aqueous solution of hydrolyzing alkoxysilane.
On the other hand, there are many proposal for polishing compositions for polishing the surface and edges of semiconductor wafers such as a silicon wafer or semiconductor device substrate having a metal film, an oxide film, a nitride film, etc. formed thereon.
The polishing composition containing the silica abrasive particles as a main ingredient is given as a solution that contains alkali components in general. The polishing mechanism can be described by the combination of chemical action by the alkali components, specifically chemical corrosion against the surface of silicon oxide film, metal films or the like, and mechanical abrasion by the silica abrasive particles. More specifically, the corrosion by the alkali components produces a thin and soft corrosion layer on the surface of an object product to be polished such as a wafer. It is estimated that the resulting corrosion layer is removed by the mechanical abrasion of the fine abrasive particles. It is considered that polishing may proceed by repeating these steps. After polishing, the silica abrasive particles and alkali components are removed from the surface and edge polished in a cleaning step. A problem that the abrasive particles remain on the wafer surface in the cleaning step has been pointed out. It is possible to largely improve such a state that the abrasive particles remain on the wafer surface by selecting properly polishing conditions or cleaning processes. On the other hand, polishing rate is largely lowered and the cleaning process becomes complicated. The problem has not yet been solved.
Polishing compositions comprising an alkali agent except sodium have been conventionally proposed for mirror-polishing a semiconductor wafer. For example, JP2-146732A discloses colloidal silica containing ethylenediamine. JP6-53313 discloses a method for polishing device wafers using an aqueous solution containing ethylenediamine pyrocatechol and silica fine powder. JP 9-193004 A discloses a polishing composition prepared by dispersing fumed silica having an average particle diameter of 5 to 30 nm in a KOH aqueous solution and a method of producing the polishing agent. JP 3-202269 A describes a polishing slurry of colloidal silica prepared by removing sodium through cation exchange. JP 3-202269 A proposes an addition of amine as a polishing promoter and quaternary ammonium salts as a bactericide. JP 2002-105440 A describes use of specific amine. JP2003-89786 discloses high-purity colloidal silica for polishing that is prepared using tetramethylammonium hydroxide in place of sodium hydroxide as an alkali agent used in the step of growing colloidal silica particles, and is substantially free of sodium.
A production process of the colloidal silica described in JP 1-317115 A includes a step of adding a water soluble calcium salt, a water soluble magnesium salt, or a mixture thereof, which remains in the product of the colloidal silica as an impurity. A production process of the colloidal silica described in JP 4-187512 A includes a step of adding a water soluble aluminum salt, which remains in the product of the colloidal silica as an impurity. The colloidal silica described in JP 11-60232 A and JP 2001-48520 A preferably has high-purity, since the silica particles are prepared using an organic silicon compound as raw material. However, the colloidal silica has disadvantageous aspects such as removal of by-product alcohol, high cost, and the like.
When ethylenediamine is used in such cases as JP 2-146732 A and JP 6-53313 A above, the harmful properties of ethylenediamine are problematic. Although KOH is used in JP 9-193004 A, the corrosive power of KOH is extremely small compared with that of NaOH, and the use of KOH offers only a little improvement in corrosion. As described on page 7 of JP 3-202269 A, colloidal silica having low sodium content uses amine as a polishing promoter, and the small amount of quaternary ammonium salts added as a bactericide also has the effect of polish promotion. Examples describe use of aminoethyl ethanolamine and piperazine as amines. A Recent study has revealed that amines cause metal contamination on wafers, especially copper contamination on wafers, due to its metal chelate formation action. JP 3-202269 A describes that KOH is to be used for pH adjustment, and the reduction of sodium amount is a problem to be solved. JP 2002-105440 A describes the risk of wafer contamination caused by aminoethyl ethanolamine. The colloidal silica described in JP 2003-89786 A is an extremely preferable polishing agent because the colloidal silica does not have sodium in its aqueous phase, on its particle surfaces, or inside its particles. However, silica particles having a large of long axis to short axis ratio have not been obtained.